[Mr Ian David Sharp]

Hi Kevin,

The first thing to bear in mind is that bias signal always needs to be subtracted during calibration. However, the thing to also remember is that *all* images contain the bias signal and this includes your darks and flats.

The traditional CCD workflow is to take separate bias, dark, and flat frames, then explicitly subtract the bias from darks, flats, and
lights. This works well with CCD’s because they are very well behaved in terms of their linearity.

CMOS cameras, however, have certain non-linearities with short exposures and dark frames (amp glow). (a lot of new CCD cameras seem to have all but eliminated amp glow).

To calibrate CMOS images, we don’t need to take separate bias frames, but rather keep the bias in the darks and flats and the bias subtraction is done when calibrating the flats with dark flats and the lights with their darks. So, it is not that bias is not used, but rather how it is ultimately subtracted.

If you use PixInsight and the excellent WBPP (Weighted Batch Pre Processing) script, then all the settings are nicely set up for you. You just need to feed it with your darks, flats and flat-darks (yes, take dark frames to match the exposure of your flats as well as your lights). The only time that PixInsight needs Bias frames is in the situation where you have not taken dark frames to match your light frames. In this situation, the bias frames are used to scale your darks to match. So, for example, if you have 300s darks in your library and you have taken some 360s lights, the program will scale the 300s master dark to produces a scaled-dark. This is not ideal but works very well.

In summary, take darks for all the exposures and temperature combinations you are likely to use. Take flats for all of your filters and also take flat-darks to match the flat exposures. I’m assuming here that you have chosen your Gain and Offset values. These must be kept the same for all lights and calibration frames.

Adam Block explains all in great depth here: https://www.youtube.com/watch?v=WzEpygFGbN0

Hope that helps!
Ian.

[Mr Ian David Sharp]

Hi Kevin,

Section 3.3.10 in the AAVSO CCD/CMOS Photometry guide is very informative with regards to the Gain and Offset settings in CMOS cameras:

https://www.aavso.org/sites/default/files/publications_files/ccd_photometry_guide/CCDPhotometryGuide.pdf

Once you choose a gain and offset, just stick with it for all lights and calibration files.

With regards to temperature, just go with a value that your camera can cool to reliably. I use both -10C and -20C with my CCD. I use -10 in the Summer months when I find the camera can’t cope with -20. I have a QHY268C CMOS camera which I image with and I find that it can’t get down reliably cooler than -15C, so I tend to stick with -10.

Binning – I use both 1×1 and 2×2 for fainter targets.

Cheers
Ian.

[Mr Ian David Sharp]

Hi Kevin,

I forgot to mention that I used an Alnitak Flip-Flap on my 104mm refractor setup at PixelSkies in Spain. See photos and details here:

http://www.astro-sharp.com/2021/10/23/my-new-remote-setup-in-spain/

This was brilliant because I could control it remotely. I have recently replaced the OTA with a C11 and now I have to ask the folks there to place a light panel on to the C11 when I need to refresh my flats (which is not very often).

Cheers
Ian.

[Mr Ian David Sharp]

Hi Kevin,

For what size OTA?

I use LED flat panels for my kit (is that what you mean by a pad?). I have an A3 size panel which covers my C11 ok. I have a smaller Pegasus Astro Flatmaster panel which covers my refractors and is dimmable and USB controllable. There are lots of choices of LED Flat panels out there now.

These look nice: https://www.widescreen-centre.co.uk/aurora-flatfield-panels.html

Cheers
Ian.

[Mr Ian David Sharp]

Hi all,

It looks to be slowly dimming again. I got Vmag = 11.62 and Bmag = 12.42 last night. See attached.

Cheers
Ian.

Attachments:

1. 

   V426-Sge-20231019.png
   

[Mr Ian David Sharp]

Thanks for the info Gary,

I managed a decent set of R, V and B last night from Spain. V mag was coming out at around 14.6 with R at about 14.2.

All uploaded into the dB.

Cheers
Ian.

[Mr Ian David Sharp]

Hi all,

I got some nice results last night in B and V – 10 images of each. I get 11.43 in V and 12.16 in B and it looks to be brightening from David and Gary’s recent points. See attached.

Cheers
Ian.

Attachments:

1. 

   V426-SGE.jpg
   

[Mr Ian David Sharp]

Thanks for the heads up David,

I’ve set up a run from Spain in B and V tonight…

Cheers
Ian.

[Mr Ian David Sharp]

Hi all,

My B mag. measurements from last night suggest a bit of a rise in the past few days. Hope to check this again tonight.

Attachments:

1. 

   T-Crb-20230920-ids.jpg
   

2. 

   20230920_T-CRB_B_PixelSkies-C11_IDS_plot_points.png
   

[Mr Ian David Sharp]

Wow! Just been watching Brad Schaefer’s presentation. This is so exciting! Now I understand why the call for B-band photometry.

Marvellous!

Ian.

[Mr Ian David Sharp]

Jeremy Shears wrote:

They said there would be a video available shortly.

Here it is: https://fb.watch/mZhv9T6ax3/

Cheers
Ian.

[Mr Ian David Sharp]

Gary Poyner wrote:

Certainly looks like it Ian. My efforts attached.

Hi Gary,

Excellent!

Here’s my update as of last night. I had three rainy nights in a row in Spain!

Cheers
Ian.

Attachments:

1. 

   SN2023ixf_05_Sep_ids.jpg
   

[Mr Ian David Sharp]

Many thanks Andrew and Kevin,

I’m coming around to the idea!

Best Regards
Ian.

[Mr Ian David Sharp]

Thanks Jeremy,

I managed to get an 11th mag comparison star in the FOV and that has improved things.

Here’s another 2 hour B run from last night.

Cheers
Ian.

Attachments:

1. 

   T-Crb-20230825-ids.jpg
   

[Mr Ian David Sharp]

Hi Jeremy,

I’ve been monitoring T CrB for a couple of months in R, V and B and have noticed much more variability in B, but it has not made any significant shifts until last night when I managed a longer run over about 2 hours. See the attached graph.

The problem I have so far is that my reference and check stars are a good 2 mags. fainter because of the tight FOV of my C11 setup. I’ll try and offset the FOV a tad to fit in a brighter ref star or two.

Cheers
Ian.

Attachments:

1. 

   T-Crb-20230824-ids.jpg
   

[Mr Ian David Sharp]

Kevin West wrote:

Are these suitable to get started in CMOS photometry. Does anyone in BAA have experience of using them?

Hi Kevin,

Yes, I use these filters, but I only use the R, V and B. I would forget ‘U’, but the ‘I’ can be useful. I use them with a CCD camera, but no problems with a CMOS camera.

As well as photometry, I also use them for colour imaging as there is not enough room in my 7-pos filter wheel for a standard set of RGB filters. I have the following filters: L, R, V, B, Ha, OIII, SII, and this works very well for everything.

Best Regards
Ian.

• This reply was modified 2 years, 2 months ago by Mr Ian David Sharp.

[Mr Ian David Sharp]

Hi all,

Last night’s results seem to suggest the start of a plateau? See attached graph.

Best Regards
Ian.

Attachments:

1. 

   SN2023ixf_16_Aug_ids.jpg
   

[Mr Ian David Sharp]

Jane Clark wrote:

In addition, I think we can make your “It’s turned back up again!” statement a little more quantitative.

Hi Jane,

Yes, of course my comment was conversational, if a bit glib!

I’ve sent you a PM with the Excel file attached.

Ian.

[Mr Ian David Sharp]

Robin Leadbeater wrote:

I see from the light curve the slow steady decline (~2 magnitudes in V over 70 days) has come to an end and the brightness is now dropping rapidly (~1.5 magnitudes in 12 days)

Hi Robin,

Yes, and it’s turned back up again! See attached graph as of last night’s data…

Cheers
Ian.

Attachments:

1. 

   SN2023ixf_14_Aug_ids.jpg
   

[Mr Ian David Sharp]

Hi all,

With regard to may last post. New data from last night reveals that the slow decline continues!

Ian.

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